As is well known and understood, one of the major concerns of designers of antenna system communications links is the elimination or reduction of external interference sources such as jamming, self-interference, atmospheric noise, and man-made noise. Also as is well known, most arrangements which attempt to resolve these problems of external interference do so in a relatively complex manner, often utilizing very large directional antennas and/or with antennas having hundreds or even more elements. This problem of external interference is particularly prevalent in the area of mobile communications systems where omnidirectional antennas are employed, because of the large number of users operating in the same frequency band and because of multipath. Use of very large directional antennas in mobile communications, moreover, is almost physically impossible and economically impractical.
A new concept for eliminating interference in a communications system utilizing a plurality of transmission links is shown and described in U.S. Pat. No. 4,275,397, entitled, "Interference Cancelling Random Access Discrete Address Multiple Access System", which issued to Frank S. Gutleber, the present inventor, on June 23, 1981. The system disclosed therein utilizes orthogonal multiplexing in conjunction with a receiver antenna configuration comprised of an omnidirectional antenna and a notch antenna at the receiving end of the transmission link to cancel interference arriving from all directions except over the narrow beamwidth's notch or null formed by the notch antenna.
Accordingly, it is an object of the present invention to eliminate external interference at the transmitting end of a transmission link.
Another object of the invention is to minimize the interference caused by radiating sources by effectively radiating only in the direction of the desired signal.
Still a further object of the invention is to provide an interference cancelling transmitter operating in a dense communications environment.
And yet another object of the invention is to provide electromagnetic compatability and low probability of intercept in a multiple user communications system.